I'm not so sure on this one. I'm up in the air about it. I can see where you're coming from, but at the same time, I can see where people are going to say that a 900lb spartan shouldn't be able to sprint endlessly. BF3 is designed around a lot of vehicle play, especially on the larger maps. If you don't get into a vehicle, you're kinda screwed. So, to counter that, you add in an unlimited sprint feature, and now somebody that doesn't get into a vehicle can get to an area quicker, but still be late to the party. I liked that feature, but at the same time, wondered why my normal human could sprint endlessly across 500 meters and not be winded, and then immediately sprint again if he wanted to.
Just remember: Spartan MJOLNR (sorry if I spelt that wrong) armor is power assisted, and spartans are pumped full of genetic steroids. Also not a surprise that spartans with their armor can accelerate to 60+ km per hour.
I think I just replied wrong...